Amnesty said sex work and human trafficking were two 'very complex social phenomena' that required different laws. Photograph: Alamy

Amnesty International has urged Northern Ireland's politicians to ditch plans to criminalise the purchasing of sex.

The human rights organisation wants a clause contained in a bill against human trafficking to be excised because it argues it would create a "hierarchy of criminal liability" among sex workers.

Clause six of the bill would make it a criminal offence to buy – but not sell – sex and is based on Sweden's model. Lord Morrow, a Democratic Unionist member of the Stormont assembly who also sits in the House of Lords, has been trying to force the bill through the devolved parliament; anti-prostitution campaigners are agitating for the purchasing of sex to be criminalised in the Irish Republic as well.

Amnesty stressed it was not taking sides on the debate over sex work and prostitution, but said sex work and human trafficking were "two very complex social phenomena" that required different laws.

Northern Ireland's justice minister, David Ford, has expressed serious reservations about the legislation and has asked his officials to put together a report on research into the issue.

Grainne Teggart, Amnesty's Northern Ireland campaigner, said: "We recommend that our political parties remove clause six from the bill and that planned research into sex work by the Department of Justice is used to inform future policy, which should establish the degree to which legislation – together with educational, social, cultural and other measures – could serve to reduce the demand that fuels trafficking, including for the purpose of sexual exploitation." She said the global human rights organisation neither supported nor endorsed any specific state response to prostitution, such as the "Swedish model".

Teggart said: "It is claimed this clause will help protect sex workers by shifting the criminal liability away from them as the seller of sexual services, to the purchaser. In reality, it fails to do this and provides no exploration of, or guarantees against, the potential unintended consequences of such a move. It is clear that many others, including the police, share our concerns on the risk of potential negative effects."

"In effect, clause six would introduce a hierarchy of criminal liability amongst those engaged in the selling of sexual services, many of whom may be vulnerable, with some remaining at risk of prosecution and others not."

Morrow's bill has won support across the assembly's traditional unionist/nationalist divide, although the second-largest party, Sinn Féin, has reserved judgment.

Ford told the Guardian this month he was "far from convinced" that the Swedish law would work because it relies heavily on mobile intercepts. Ford pointed out that the use of the surveillance of mobile phone calls from so-called punters to sex workers, is rarely used in other aspects of law, including cases against republican and loyalist terror groups.

Ford said only a cabinet minister in London would have the power to sign off every single spying operation on the customers of sex workers. The use of mobile and telephone intercepts do not lie in the power of the devolved administration in Belfast.

Many sex workers' groups argue that criminalising the purchase of sex could push the trade further underground and put prostitutes in even greater danger.

Critics of Morrow's bill have pointed out that clause six does not remove any of the laws drawn up in 2008, which criminalise the sex worker. They have claimed that if the clause passes through the parliament, sex workers would have to give evidence against those attempting to buy sexual services from them. This would mean the sex worker giving evidence would have to admit she or he had committed other sexual offences contained in the 2008 legislation, such as managing or assisting in managing a brothel.